1. Field of the Invention
The present invention relates to a method for determining an illumination source with optimized depth of focus. In particular, the present invention relates to a method for determining an illumination source with optimized depth of focus by calculating an optimal correlation ratio.
2. Description of the Prior Art
An image DOF (depth of focus) is an important index in lithography processes. When the DOF is too small, in-line process control will become very difficult. Accordingly, simulations are always done in advance to optimize the illumination sources for specific patterns because DOF is a highly decisive value for judgment. When there are thousands of possible source split conditions, analysis and determination of various possible simulation results becomes an arduous and heavy burden.
Every source condition has to perform a DOF calculation for judgment. As shown in FIG. 1, the DOF calculation involves an FEM (Focus-Energy-Matrix) simulation for obtaining a Bossung curve chart and then calculates a DOF value from the chart, as shown in FIG. 2. Simulated FEM data has to cover the specifications of CD target for getting complete DOF values. Because every source condition has different DOF values, the dose (threshold) and focus boundaries need to be determined for each condition in order for the FEM simulation to be carried out.
An FEM usually needs 5 to 10 splits for dose and focus individually to form a complete Bossung curve. The total number of simulation times is at least 25 to 100 for each source condition. In view of the above problems, a simpler approach is needed to determine an illumination source with an optimized depth of focus, as shown in FIG. 3.